1. Field of the Invention
The present invention relates to the production of synthetic polyamide compositions having a high degree of whiteness and color stability and improved molding cycle time.
2. Description of Related Art
Polymerization of suitable diamines with dicarboxylic acids to form polyamides is well known in the art and is of considerable commercial significance. Polyamides have a variety of uses. One important commercial use is resin for molding articles, especially articles that require toughness and the ability to withstand heat.
Nucleants have often been used to improved the molding cycle time or crystallization rate of polyamides. For example, U.S. Pat. No. 3,080,345 discloses using as a nucleating agent sodium phenylphosphinate, sodium isobutylphosphinate, magnesium oxide, mercuric bromide, mercuric chloride, cadmium acetate, lead acetate, or phenolphthalein. U.S. Pat. Nos. 3,585,264 and 4,866,115 also disclose using nucleating agents for improving the rate of crystallization of polyamides.
Hypophosphite compounds have been used as catalysts for polymerization of the polyamides, for example in U.S. Pat. Nos. 3,860,558; 3,173,898; and 3,691,131. In U.S. Pat. Nos. 3,860,558 and 3,691,131, metal hypophosphites are used along with a hindered phenolic compound as an antioxidant. In U.S. Pat. 3,173,898, hypophosphites of certain metals are used in small concentrations. The use of hypophosphites in the polymerization process also gives the polyamide a greater degree of whiteness and color stability.
One problem in polyamide manufacturing is that most nucleants are rendered much less effective in the presence of hypophosphite. Therefore, a need exists for polyamide compositions and manufacturing processes that will have the advantages imparted by the use of a nucleant, such as reduced molding cycle time, while still permitting the use of hypophosphites.